mutable records

[Tom Pledger]

Scott J. writes:
 :
 | Sill I want to make objects packed with their objects and
 | functions. Doesn't mean that I have to use existential data types?

Sometimes you can avoid using existentials by making all your
object-updating functions return the post-update object explicitly.

For example:

    module FiniteMap where
    data FM k v
        = FM {set    :: k -> v -> FM k v,
              get    :: k -> Maybe v,
              toList :: [(k, v)]}

    module RedBlack(empty) where
    import FiniteMap
    empty :: Ord k => FM k v    -- a finite map implemented as a red/black tree
    ...

In this case, the red/black tree itself is effectively a private field
of the object.  The privacy is achieved by *not* making the tree a
field of the FM record.  Instead, it gets passed around as the
parameter of an auxiliary function:

    -- module RedBlack, continued
    empty = fm emptyRedBlackMap
    fm rb = FM {set    = \k v -> fm (extendRedBlackMap rb k v),
                get    = \k   -> ...,
                toList = redBlackMapToList rb}

HTH,
Tom